20200501 A command is a function (Section 3.5) for which we are generally more interested in the actions or side-effects that are performed by the command rather than the value returned by the command. For example, the base::library() command is run to attach a package from the library (the action) and consequently modifies the search path that R uses to find commands (the side-effect).
# Load package from the local library into the R session. library(rattle)
A command is also a function and does in fact return a value. In the above example we do not see any value printed. Functions in R can be implemented to return values invisibly. This is the case for base::library(). We can ask R to print the value when the returned result is invisible using the function base::print().
# Demonstrate that library() returns a value invisibly. print(library(rattle))
##  "readr" "rattle" "bitops" "tibble" "tidyr" "stringr" ##  "ggplot2" "dplyr" "magrittr" "stats" "graphics" "grDevices" ##  "utils" "datasets" "methods" "base"
We see that the value returned by base::library() is a
vector of character strings. Each character string names a package
that has been attached during this session of R. Notice that the
vector begins with item
 and then item
on the second line, and so on. We can save the resulting vector into a
variable (Section 3.18) and then index the variable to
identify packages at specific locations within the vector.
# Load a package and save the value returned. <- library(rattle) l # Review one of the returned values. 7]l[
##  "ggplot2"
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